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Identification and partial characterization of two steroid 5 alpha-reductase isozymes in the canine prostate
P N Span1, J A Schalken, F G Sweep
1Department of Medicine, University Hospital St. Radboud, Nijmegen, The Netherlands. p.span@lev.azn.nl
Background:
The dog has been extensively used as an in vivo model to test the pharmacokinetics and effects on pathological prostatic growth of 5 alpha-reductase inhibitors. However, no information is available on the existence or characteristics of canine 5 alpha-reductase isozymes.
Methods:
The 5 alpha-reduction of testosterone is analyzed in dog prostatic homogenates. Three human-specific inhibitors are tested for their activity against dog 5 alpha-reductase.
Results:
Two pH optima of 5 alpha-reductase activity in dog prostatic homogenates are described, comparable to the pH optima of rat and human 5 alpha-reductase isozymes. Kinetic analysis of 5 alpha-reductase enzymatic activity at pH 7.0 revealed isozymes with a low apparent affinity constant (Km = 2.67 nM) and a high apparent affinity constant (Km = 1.23 microM). These apparent affinity constants compare favorably to the human and rat isozymes types II and I, respectively. The human type II inhibitor finasteride selectively inhibited the low Km isozyme, whereas the human type I inhibitor MK386 preferentially inhibited the high Km isozyme. The human type I inhibitor LY306089 was nonspecific for the dog isozymes.
Conclusions:
We postulate that the high and low Km isozymes described here represent the dog type I and type II 5 alpha-reductase isozymes, respectively.